|Leak High Frequency
The Leak Ribbon loudspeaker for high frequencies was never put into production. However it did make it into a prototype stage, and as such it played a role in a research programme on suitable transducers for a high quality loudspeaker.
This research was conducted in the mid 1950's, and some of the results were published by Leak in the paper referenced below. This paper focusses heavily on the search for a piston-action loudspeaker, and the ribbon loudspeaker shown at the left was a contender for consideration (as the actuating force is applied evenly across the surface of the ribbon diaphragm, thus preventing diaphragm "break-up" found with cone speakers at high frequencies).
The ribbon speaker features a very light aluminium alloy diaphragm 2 inches long, and 3/8 inch wide, and only a fraction of a thousand'th of an inch thick. A powerful magnetic field is applied across the face of the ribbon diaphram, which has the signal current applied along its length. No mention is made of an associated matching transformer, but one was probably required to match the very low impedance of the ribbon (typically less than 0.25 Ohm) to the amplifier.
A strong magnetic field was required to obtain sufficient efficiency. The cost of such magnets limited the size of the unit (since a wide range unit needed a long ribbon, and hence more magnets, and so more cost). So the small size restricted the frequency range to 3kHz and above. A horn was also added to improve efficiency.
The measured performance of the ribbon showed a very smooth frequency response being with +/- 3dB from 2kHz to 20kHz, both on-axis, and 30 dgress off-axis. Leak noted the excellent transient response of the unit, and stated that "the ribbon loudspeaker has been the most faithful reproducer available in the past".
It seems that Leak rejected further work on this prototype on the basis of cost. Leak were searching for a suitable high-frequency unit to operate with a 15 inch bass unit as a two-way system. The cross-over frequency required was around 1kHz for the 15 inch bass unit, and the ribbon could not quite make it down to 1kHz with reasonable cost. These days of course, a three-way system would be used, and the ribbon would perform very well indeed from 3kHz upwards. However H.J Leak regarded this as "rather costly and cumbersome" in the mid-1950's.
Reference and Acknowledgments:
Technical information and photo of Leak Ribbon prototype speaker from Journal of British Institute of Radio Engineers, December 1955: "High Fidelity Loudspeakers: The Performance of Moving Coil and Electrostatic Transducers." by H.J.Leak.